全文获取类型
收费全文 | 1805篇 |
免费 | 53篇 |
国内免费 | 10篇 |
专业分类
化学 | 1073篇 |
晶体学 | 14篇 |
力学 | 43篇 |
数学 | 430篇 |
物理学 | 308篇 |
出版年
2020年 | 16篇 |
2019年 | 18篇 |
2018年 | 12篇 |
2017年 | 16篇 |
2016年 | 32篇 |
2015年 | 32篇 |
2014年 | 33篇 |
2013年 | 92篇 |
2012年 | 80篇 |
2011年 | 94篇 |
2010年 | 53篇 |
2009年 | 44篇 |
2008年 | 84篇 |
2007年 | 81篇 |
2006年 | 97篇 |
2005年 | 93篇 |
2004年 | 65篇 |
2003年 | 67篇 |
2002年 | 84篇 |
2001年 | 21篇 |
2000年 | 32篇 |
1999年 | 26篇 |
1998年 | 28篇 |
1997年 | 30篇 |
1996年 | 27篇 |
1995年 | 15篇 |
1994年 | 31篇 |
1993年 | 27篇 |
1992年 | 21篇 |
1991年 | 22篇 |
1990年 | 19篇 |
1989年 | 17篇 |
1988年 | 13篇 |
1987年 | 18篇 |
1986年 | 18篇 |
1985年 | 20篇 |
1984年 | 30篇 |
1983年 | 21篇 |
1982年 | 30篇 |
1981年 | 33篇 |
1980年 | 28篇 |
1979年 | 23篇 |
1978年 | 30篇 |
1977年 | 23篇 |
1976年 | 20篇 |
1975年 | 17篇 |
1974年 | 20篇 |
1973年 | 19篇 |
1972年 | 14篇 |
1971年 | 13篇 |
排序方式: 共有1868条查询结果,搜索用时 218 毫秒
81.
Snežana Jovanović Ralph Puchta Olivera Klisurić 《Journal of Coordination Chemistry》2016,69(5):735-747
The crystal structure of K[PtCl3(caffeine)] was determined. The coordination geometry around platinum is square-planar formed by N9 of the caffeine ligand and three Cl? ions. The bond lengths and angles of K[PtCl3(caffeine)] were compared with those reported for [PtCl3(caffeine)]? and K[PtCl3(theobromine)]. At the level of the statistical significance of the data we have compared, no differences in the bond distances and angles for any of these compounds were noticed. Weak interactions between K+ and Cl? are responsible for the formation of 1-D polymeric chains in the crystal structure of the complex. The interactions of K[PtCl3(caffeine)] with inosine (Ino) and guanosine-5′-monophosphate (5′-GMP) were studied by 1H NMR spectroscopy at 295 K in D2O in a molar ratio of 1 : 1. The results indicate formation of the reaction product [PtCl3(Nu)] (Nu=Ino or 5′-GMP) with the release of caffeine from the coordination sphere of the starting complex. The higher stability of the bond between the Pt(II) ion and Ino or 5′-GMP compared to the stability of the platinum–caffeine bond is confirmed by density functional theory calculations (B3LYP/LANL2DZp) using as models 9-methylhypoxanthine and 9-methylguanine. 相似文献
82.
83.
84.
85.
Stereospecific Palladium-Catalyzed Acylation of Enantioenriched Alkylcarbastannatranes: A General Alternative to Asymmetric Enolate Reactions 下载免费PDF全文
Chao-Yuan Wang Glenn Ralph Joseph Derosa Prof. Dr. Mark R. Biscoe 《Angewandte Chemie (International ed. in English)》2017,56(3):856-860
We report the development of a Pd-catalyzed process for the cross coupling of unactivated primary, secondary, and tertiary alkylcarbastannatrane nucleophiles with acyl electrophiles. Reactions involving optically active alkylcarbastannatranes occur with exceptional stereofidelity and with net retention of absolute configuration. Because the stereochemistry of the resulting products is entirely reagent-controlled, this process may be viewed as a general, alternative approach to the preparation of products typically accessed via asymmetric enolate methodologies. Additionally, we report a new method for the preparation of optically active alkylcarbastannatranes, which should facilitate their future use in stereospecific reactions. 相似文献
86.
87.
Ralph A. Zingaro Hema M. K. K. Pathirana Joseph H. Reibenspies Edward A. Meyers 《Phosphorus, sulfur, and silicon and the related elements》2013,188(1-4):91-99
Abstract Tellurium tetrahalides undergo reaction with glycols to yield three different products: O,O'-dioxotrihalotellurates; bis(alkoxy)dihalotellurium (IV) compounds and hexahalotellurates. The course of the reaction appears to be determined primarily by the nature of the glycol. The structure of dichlorobis(cis- 2-hydroxycyclohexyloxy)tellurium(IV) has been determined crystallographically 相似文献
88.
Edward A. Meyers Cynthia A. Applegate Ralph A. Zingaro 《Phosphorus, sulfur, and silicon and the related elements》2013,188(2-4):317-327
Abstract The reaction between 10,10′-bis(phenoxarsine) oxide (I) and HI gives 10-iodophenoxarsine. The latter, on treatment with H2Se give 10,10′-bis(phenoxarsine) selenide (II). The crystal structures of I and II have been determined from single crystal X-ray data. The unit cell for I is monoclinic, P21/c (No. 14) with a = 15.976(3) Å, b = 10.582(2) Å, c = 12.581(2) Å, β = 111.70(1)° V = 2018.6 Å3; d(calc.) = 1.65Mg/m3 at 23°C for four molecules per unit cell. From 3279 reflections for which I>0.5σ(I), F>σ(F), R = 0.041 with anisotropic thermal parameters for all non-hydrogen atoms and with fixed positions and thermal parameters for hydrogens. One of the phenoxarsina rings deviates from planarity by approximately 5° while the other deviates by more than 24°. The (As[sbnd]O) distances are 1.810(3) and 1.821(3) Å for the flat and bent ring and the (As[sbnd]O[sbnd]As) angle is 122.3(1)°. The bond distances to As and O from C are nearly the same for both rings, but the bond angles with As and the ring O as the apex are systematically larger for the flat ring. For II the unit cell is triclinic, P1 (No. 2) with a = 9.368(1) Å, b = 14.089 Å, c = 9.269(2) Å, α = 111.37(2), β = 113.11(2), γ = 74.76(1); V = 1037.5 Å3, d(calc) = 1.81 Mg/m3 for two molecules per unit cell at 23°C. From 2945 reflections for which I > 0.5σ(I), F > σ(F), R = 0.055 with anisotropic thermal parameters for all non-hydrogen atoms and with fixed positions and thermal parameters for hydrogen. One of the phenoxarsina rings deviates by 3° from planarity and the other by 8°. The (As[sbnd]Se) bond distances are 2.416(1) and 2.406(1) Å. The (As[sbnd]Se[sbnd]As) bond angle is 96.66(4)° and the corresponding (As[sbnd]C) and (C[sbnd]C) distances in the two rings are nearly the same. In comparison with I, the angles with As or O as the central atoms are about the same in both rings of II. 相似文献
89.
A Henk. Van Doren Ralph Van Der Geest Cor A. Keuning Richard M. Kellogg Hans Wynberg 《Liquid crystals》2013,40(1):265-283
Abstract Ten homologous series (n-butyl through n-decyl) of aldose S,S-acetals (D-glucose, D-galactose, D-mannose, L-rhamnose, 2-deoxy-D-glucose, D-xylose, D-lyxose, D- or L-arabinose, D-ribose and 2-deoxy-D-ribose) have been prepared. Most of these compounds form thermotropic liquid crystals, the exceptions being the entire L-rhamnose series and some of the derivatives with the shortest alkyl chains. All of the compounds have been investigated with differential scanning calorimetry and polarization microscopy. Some temperature dependent powder X-ray data are also presented. A model is proposed which correlates the carbohydrate configuration with the melting behaviour. On the basis of now available behaviourial characteristics, visual inspection, mixing experiments and precedent, the mesophase is identified as smectic Ad, the partially overlapping carbohydrate moieties being in the centre and the aliphatic chains pointing outward at an angle of about 62°. Despite the intrinsic chirality of all the carbohydrate mesogens, no evidence for chiral mesophases was found, not even after introduction of a homochiral branched alkyl chain. 相似文献
90.
Mareike Prüfer Christian Wenger Frank F. Bier Eva-Maria Laux Ralph Hölzel 《Electrophoresis》2022,43(18-19):1920-1933
Dielectrophoresis (DEP) is an AC electrokinetic effect mainly used to manipulate cells. Smaller particles, like virions, antibodies, enzymes, and even dye molecules can be immobilized by DEP as well. In principle, it was shown that enzymes are active after immobilization by DEP, but no quantification of the retained activity was reported so far. In this study, the activity of the enzyme horseradish peroxidase (HRP) is quantified after immobilization by DEP. For this, HRP is immobilized on regular arrays of titanium nitride ring electrodes of 500 nm diameter and 20 nm widths. The activity of HRP on the electrode chip is measured with a limit of detection of 60 fg HRP by observing the enzymatic turnover of Amplex Red and H2O2 to fluorescent resorufin by fluorescence microscopy. The initial activity of the permanently immobilized HRP equals up to 45% of the activity that can be expected for an ideal monolayer of HRP molecules on all electrodes of the array. Localization of the immobilizate on the electrodes is accomplished by staining with the fluorescent product of the enzyme reaction. The high residual activity of enzymes after AC field induced immobilization shows the method's suitability for biosensing and research applications. 相似文献